In modern helicopters, the trend towards main rotor systems which have low inertia reduces the amount of stored energy in the rotor system and causes the rotor to be more susceptible to large transient speed excursions during certain flight maneuvers, e.g., while executing left or right roll maneuvers. Such main rotor speed excursions, working in conjunction with other flight characteristics of helicopters, may result in unbalanced torque causing the nose of the aircraft to deviate from the desired heading. This undesirable deviation in the aircraft heading may cause an increase in pilot workload, frequently at critical times, or saturation of the aircraft stability augmentation system, or both.
Typically, the helicopter main rotor and a tail rotor are driven by a common gear mechanism. The tail rotor primarily functions as a torque compensating device to counteract the torque effect of the main rotor. Secondarily, the tail rotor is a means for controlling the angular movement of the helicopter about its vertical (yaw) axis, thereby controlling the heading of the helicopter. It is common at low air speeds to utilize tail rotor yaw to control the heading of the aircraft; but at high speeds, it is common to employ roll to effect a turn, utilizing tail rotor yaw only to coordinate the turn.
It is well known in the art that a roll maneuver is performed by changing the direction of main rotor thrust, e.g., by changing the pitch of each main rotor blade individually as it rotates through one rotation cycle. The pilot operated control that accomplishes this change is known as the cyclic pitch stick. To effect a roll maneuver, the cyclic pitch stick is moved laterally to the left to accomplish a left roll maneuver and laterally to the right to accomplish a right roll maneuver. In modern helicopters, the cyclic stick may be responsive to the amount of force applied to the stick rather than the movement of the stick to input lateral cyclic command signals. Typically, the helicopter is provided with a lateral cyclic pitch control system which controls the roll axis of the helicopter during a roll maneuver and provides automatic coordinated turns by combining the desired amount of roll with a correct amount of yaw, as is well known in the art. Examples of systems that provide coordinated turns are U.S. Pat. Nos. 4,003,532, 4,067,517, 4,206,891 and 4,392,203. In an aircraft where automatic turn coordination is provided, control of the main rotor cyclic pitch is provided by both the pilot operated cyclic pitch stick and the lateral cyclic pitch control system.
During certain roll maneuvers, undesirable rotor speed excursions may occur. For example, during a high rate left lateral cyclic pitch command (for rotor systems which rotate counterclockwise), the main rotor exhibits a rise in torque during the left roll maneuver. Similarly, during a high rate right lateral cyclic pitch command, the main rotor exhibits a decrease in torque during the right roll maneuver. These torque variations continue until the rotor settles into a steady state (rate) condition. It is believed, though not completely understood, that gyroscopic forces together with air inflow and rotor wake are responsible for these torque changes.
The increased main rotor torque during a left roll maneuver, coupled with the sluggishness of the helicopter's automatic fuel compensation system, result in undesirable variations in main rotor speed and variations in the heading of the aircraft. Similarly, variations in main rotor speed and aircraft heading occur due to reduced main rotor torque during a right roll maneuver. In aircraft used for military purposes, a further negative effect of rotor speed and heading variations is reduced accuracy in weapons targeting.